I'm a middle child, and one of my own children is a middle child - and both of us are pretty happy with how we feel about ourselves.
I think a lot of the problem with middle children isn't with the children, themselves; but with parents' imagining things about middle children that aren't necessarily even real problems.
When all the children in a family are treated as if they're valued; when parents aren't looking for "something wrong" in their middle child; and when all children are raised in a way that doesn't foster envious, needy, personalities; there doesn't have to be much of a "middle child syndrome".
I believe that children who have rivalry with siblings (regardless of their birth order) have it because parents don't/can't give enough attention to children when they need it most (when they're babies/preschoolers). Some parents, of course, foster rivalry in other ways as well.
I was perfectly happy to be close to both my older sister and my younger brother. In fact, I saw a whole lot of advantages to being a middle child. My biggest complain about being a middle child (whether when I was a kid or today) has always been that "the world" kind of assumes middle children "have issues - when, in fact, not all do.